JP5445775B2 - Ultra-high resolution terahertz spectrometer - Google Patents

Description

  The present technology relates to terahertz spectroscopy measurement technology using terahertz electromagnetic wave pulses (hereinafter referred to as terahertz pulses). The present invention relates to a technique for spectroscopic measurement with high resolution.

  In recent years, it has become clear that various substances such as vitamins, sugars, pharmaceuticals, pesticides, prohibited drugs, plastic bombs, and cancer tissues show unique absorption spectra (fingerprint spectra) in the terahertz region. The terahertz spectroscopy used has attracted attention as a new analytical tool. The terahertz region includes various gas molecules in the atmosphere (oxygen, water vapor, ozone, carbon monoxide, hydrogen-related molecules, nitrogen-related molecules, chlorine-related molecules, sulfur-related molecules, etc.) and volatile organic compounds (VOC; toluene). , Xylene, ethyl acetate, etc.) and the like, and is expected to serve as an atmospheric environment analysis means. In these analyzes, extremely high spectral accuracy and spectral resolution are required in order to exhibit sufficient substance discrimination ability, but terahertz time domain spectroscopy (hereinafter referred to as THz-TDS) based on conventional mechanical time delay scanning. The measuring device using the method is not sufficient. Under such circumstances, there is a demand for a measurement technique that can realize extremely high spectral accuracy and spectral resolution and can greatly improve the substance identification ability based on the terahertz fingerprint spectrum.

  In a conventional measuring apparatus using the THz-TDS method based on mechanical time delay scanning, as shown in FIG. 1, a single femtosecond laser beam is converted into pump light (terahertz pulse generation) and probe pulse light (terahertz pulse). Since both are used for both (pulse detection), they are always synchronized. Therefore, in the THz-TDS method, time delay scanning is performed by a mechanical stage for optical path length scanning, cross-correlation measurement is performed while sequentially shifting the time timing at which both pulses are overlapped by the terahertz detector (pump / probe method), Finally, the time waveform of the terahertz electric field is reproduced.

FIG. 2 shows a state in which terahertz waveforms are reconstructed by performing two time-delayed scans by moving the mechanical stage and measuring three points.
In the THz-TDS method, spectroscopic measurement is performed using a frequency spectrum of an amplitude (or phase) obtained by Fourier transforming a measured time waveform of a terahertz electric field with a computer.

  FIG. 3 shows a frequency spectrum of the amplitude of the terahertz electric field obtained by performing Fourier transform with a computer from the measured time waveform of the terahertz electric field. Here, in the measured time waveform of the terahertz electric field, if the measurement time window is T, the frequency resolution of the terahertz amplitude spectrum is represented by 1 / T.

That is, the frequency resolution is determined by the measurement time window T (time delay scanning amount) of the terahertz electric field time waveform, and the moving stroke length of the mechanical stage (L = cT / 2; c is the speed of light (= 3 × 10 ⁸ m / s). ), T is limited by the measurement time window). On the other hand, the frequency range is given by the reciprocal (1 / t) of the step time interval t for sending the time delay amount.

  For this reason, the THz-TDS method based on the conventional mechanical time delay scanning is a trade-off between improving the spectral resolution and shortening the measurement time due to the use of the mechanical stage (in order to obtain a high spectral resolution, a long stage). As a result, there is a problem in that there is a limit to the compatibility between high-resolution measurement and high-speed throughput capability. For example, the spectral resolution when the measurement time is 5 minutes is about 30 GHz.

Further, since the frequency spectrum is calibrated based on the movement amount of the mechanical stage, there is a problem that there is a limit to the achievable spectrum accuracy and spectrum resolution. For example, the positioning accuracy of the mechanical stage used in the THz-TDS method is generally low, and only about 10 ⁻³ is obtained as the spectral accuracy.

In view of the problem of the THz-TDS method based on such a conventional mechanical time delay scanning, the present inventors have made it possible to perform a spectroscopic measurement of a terahertz spectrum at a high speed with a spectral resolution equal to the mode-locking frequency at the maximum. Asynchronous optical sampling type THz-TDS in which the time delay scanning is omitted has already been proposed (Patent Document 1).
As shown in FIG. 4, the proposed asynchronous optical sampling terahertz spectrometer has a femtosecond laser 1 (mode synchronization frequency = f ₁ ) and a femtosecond laser 2 with slightly different repetition frequencies (mode synchronization frequencies) of laser pulses. Each mode synchronization frequency of (mode synchronization frequency = f ₂ ) is highly stabilized and the difference between the mode synchronization frequencies (= f ₂ −f ₁ ) is constant at a certain value. Controls the mode synchronization frequency. Both laser beams are used for terahertz pulse generation pump light and probe pulse light, respectively, and a part of both laser beams is extracted to generate a trigger signal.

FIG. 5 is a schematic diagram showing how the terahertz electric field time waveform is reproduced in the proposed asynchronous light sampling terahertz spectrometer. Since the pulse periods of the generated terahertz pulse and the probe pulse light are slightly different (the terahertz pulse period is 1 / f ₁ , the probe pulse period is 1 / f ₂ , and the time interval SI is shifted to each pulse SI = 1 / f ₂ -1 / f ₁ ), the timing at which the terahertz pulse and the probe pulse light overlap is automatically shifted for each pulse.
From the state in which the terahertz pulse and the probe pulse light overlap (see (a) in FIG. 5), the overlapping timing is automatically shifted for each pulse, and the state overlaps again (see (b) in FIG. 5). The time delay scanning corresponding to the pulse period is performed once by the time required until the time (sampling time ST). The trigger signal generates a time origin signal every time the terahertz pulse and probe pulse light overlap ((a) and (b) in FIG. 5). This is used as a time origin signal.

  The signal waveform of the terahertz pulse obtained in this way is observed with time expansion based on the principle of the asynchronous optical sampling method. By converting the signal waveform of the observed terahertz pulse at the time axis scale expansion rate, it becomes possible to reproduce the electric field time waveform of the terahertz pulse of the actual time scale. A frequency spectrum of amplitude and phase can be obtained by Fourier transforming the scaled electric field time waveform. In this case, the frequency resolution does not depend on the sampling time ST, and time-delayed scanning that is always equal to the pulse period is performed, so that it is possible to realize a spectral resolution that is equal to the mode synchronization frequency at the maximum. As a result, a mechanical stage for time delay scanning can be omitted, and improvement in spectral resolution and measurement speed can be realized at the same time.

  6 and 7 show the electric field time waveform (measurement time 200 milliseconds) obtained by the proposed asynchronous optical sampling terahertz spectrometer and the power spectrum waveform obtained by Fourier transforming the waveform. Spectral waveforms can be acquired with a good measurement signal-to-noise ratio despite a short measurement time.

  In the proposed asynchronous optical sampling terahertz spectrometer, the terahertz without using a mechanical scanning mechanism based on the mode synchronization frequency and the mode synchronization frequency difference phase-synchronized with a frequency standard (for example, rubidium atomic clock). Spectral measurement is performed, and the spectral scale is scaled using the ratio of the mode synchronization frequency and the mode synchronization frequency difference. Since the frequency is a physical quantity for which the most stable standard device has been prepared, it is possible to realize ultra-precision spectroscopy that is directly traceable to the frequency standard device by controlling this with the ultra-precision laser as a reference signal. In addition, the omission of the mechanical scanning mechanism eliminates the trade-off between improving the spectral resolution and shortening the measurement time, and can realize a spectral resolution significantly higher than that of the conventional THz-TDS method by high-speed measurement.

International Publication Number WO2006 / 092874

However, the proposed terahertz spectroscopic measurement apparatus still has insufficient spectral resolution depending on the application field. In the case of the proposed terahertz spectrometer, as shown in FIG. 8, since only a single pulse electric field time waveform is acquired, the amplitude spectrum obtained by Fourier transform becomes a continuous spectrum. The waveform resolution (data plot interval) was insufficient. The same applies to THz-TDS using conventional mechanical time delay scanning.
In view of the above, an object of the present invention is to provide a terahertz spectroscopic measurement apparatus capable of further improving spectral resolution.

In order to achieve the above object, the terahertz spectrometer of the present invention includes two femtosecond laser units having slightly different repetition frequencies (mode synchronization frequencies) of laser pulses, and modes of the two femtosecond laser units. The mode-synchronized frequency control means for controlling the two femtosecond laser means so that the synchronization frequency is stabilized and the difference between the mode-synchronized frequencies maintains a predetermined constant value, and the output light of one femtosecond laser A terahertz wave radiating means that emits terahertz electromagnetic wave pulses using a photoconductive antenna or a nonlinear optical crystal, and a measurement sample by irradiating the terahertz electromagnetic wave pulses emitted from the terahertz wave radiating means to the measurement sample. Terahertz wave optical means for guiding the terahertz electromagnetic wave pulse affected by the Terahertz wave detection using terahertz electromagnetic wave pulse and probe pulse light as input probe light, and detecting electric field time waveform of terahertz electromagnetic wave pulse using photoconductive antenna or electro-optic sampling method And a part of the output light of the two femtosecond laser means, a trigger signal generating means for generating a time origin signal, and a weak electric signal output from the terahertz wave detecting means are amplified to obtain a time origin signal. In the previously proposed asynchronous optical sampling type terahertz spectroscopic measurement apparatus, comprising a signal waveform measuring means for measuring a signal waveform by detecting a signal waveform of a terahertz electromagnetic wave pulse in synchronization,
a) The signal waveform measuring means obtains an electric field time waveform of a plurality of continuous terahertz electromagnetic wave pulses (a terahertz electromagnetic wave pulse train) instead of a single terahertz electromagnetic wave pulse, and Fourier-transforms the electric field time waveform into the frequency domain. Spectrum acquisition means for acquiring a terahertz discrete multispectrum in which a frequency mode sequence composed of a fundamental wave component of a mode-locked frequency and a number of harmonic components is arranged at equal intervals;
b) mode-synchronized frequency tuning control means for shifting the mode-synchronized frequency of each of the two femtosecond laser means while maintaining a predetermined constant value of the mode-synchronized frequency difference;
c) storage means for storing each terahertz discrete multispectrum when the terahertz discrete multispectrum is shifted so as to complement the gap of the frequency mode sequence by the mode-locked frequency tuning control means;
d) means for synthesizing each terahertz discrete multispectrum stored in the storage means to generate an ultrafine terahertz spectrum;
It is set as the structure provided with.

  According to this configuration, it is possible to further improve the spectral resolution in the asynchronous light sampling terahertz spectrometer.

  In the above asynchronous optical sampling terahertz spectrometer, as shown in FIG. 8, an electric field time waveform of a single terahertz pulse is obtained in an arbitrary measurement time window within the pulse period, and this is subjected to Fourier transform. Obtain the frequency spectrum of amplitude (or phase). Since the spectrum in this case is a single phenomenon Fourier transform, the spectrum is a continuous spectrum, and the data plot interval (spectral resolution) is the reciprocal of the measurement time window.

On the other hand, when the measurement time window is enlarged larger than the pulse period, an electric field time waveform of a terahertz pulse train composed of a plurality of terahertz pulses can be obtained as shown in FIG. When a time waveform of such a repetitive phenomenon is Fourier-transformed, a spectrum waveform in which a large number of frequency mode sequences are regularly arranged at intervals of a mode synchronization frequency (pulse repetition frequency) is obtained. The terahertz band discrete multispectral structure in which the fundamental wave component of the mode-locked frequency and the many harmonic components are arranged in a comb-like shape at equal intervals in this way is called a terahertz comb. Since the terahertz comb has a discrete multispectral structure in which a large number of frequency mode sequences are arranged at mode-locked frequency intervals, the spectral resolution when this is used for spectroscopic measurement is a mode-locked frequency that is the comb interval. On the other hand, the spectral line width of each frequency mode (hereinafter referred to as comb mode) is further narrower than the comb interval.
Further, the envelope wave (envelope) of each mode component peak of the terahertz comb shows a spectrum waveform similar to the spectrum of FIG. 8, but the data plot (comb mode) interval is a mode synchronization frequency. On the other hand, the line width of the comb mode is the reciprocal of the measurement time window of the terahertz pulse train time waveform of FIG. 9, and is significantly narrower than the comb mode interval.

  Terahertz Com has features such as wide frequency selectivity, very high spectral purity, direct absolute frequency calibration, frequency multiplication function, and simplicity, so it is highly stabilized and used as an absolute frequency scale. Then, terahertz spectroscopic measurement with extremely high spectral resolution and accuracy becomes possible. In this case, since each mode of the terahertz comb is discretely distributed, the spectral resolution is a mode synchronization frequency which is a comb interval.

On the other hand, as shown in FIG. 10, if spectral measurement can be carried out while laterally shifting the comb mode so as to interpolate the terahertz comb gap (frequency region between the comb modes), the spectral resolution can be further improved. Can be improved. The frequency of the comb mode can be scanned by changing the mode synchronization frequency (f ₁ , f ₂ ) while keeping the mode synchronization frequency difference (f ₂ −f ₁ ) of the _two lasers constant. . In this case, the lateral shift amount (frequency shift) of the comb mode is the product of the frequency change amount of f ₁ (or f ₂ ) and the order of the comb mode.

  That is, by sequentially shifting the frequency of the comb mode at line width intervals so as to interpolate the gap of the terahertz comb, the terahertz comb spectrum at each comb mode position is measured, and finally synthesized, It becomes possible to obtain an ultrafine terahertz spectrum plotted at intervals of the comb mode line width. This is equivalent to continuous scanning of the frequency of a narrow line width CW (continuous oscillation) terahertz wave, so the spectral resolution in this case is the line width of the comb mode. Furthermore, further narrowing can be achieved by enlarging the measurement time window of FIG.

The terahertz spectroscopic measurement apparatus according to the present invention has a discrete distribution of terahertz combs while observing terahertz combs and narrowing the line width of the comb mode and shifting the frequency of the comb mode little by little with high accuracy. Interpolate the gap to obtain a hyperfine terahertz spectrum with signal distribution in the comb mode linewidth interval.
The spectrum acquisition means (a) can observe terahertz combs and narrow the line width of the comb mode. In addition, the comb mode is shifted little by little with high accuracy by the mode synchronous frequency tuning control means of b). According to the above-mentioned c) and d), each terahertz comb when it is shifted in multiple stages so as to interpolate the gap of the terahertz comb is synthesized, and the signal is distributed at the line width interval of the comb mode. A terahertz spectrum is obtained.
In this manner, terahertz spectroscopic measurement capable of fully covering a broadband terahertz region while having a spectral resolution equal to the line width of the comb mode can be realized.

In the b) mode-locked frequency tuning control means of the terahertz spectroscopic measurement apparatus of the present invention, the shift amount of the mode-locked frequency is a measured terahertz frequency band so that the shift amount of the comb mode is equal to the comb mode line width. It is preferable to adjust in consideration of the order of the comb mode determined from the mode synchronization frequency.
Thereby, a spectral resolution equal to the line width of the comb mode can be obtained.

The two femtosecond laser means of the terahertz spectrometer of the present invention are preferably a fiber laser, a titanium sapphire laser, or a neodymium glass laser.
Thereby, the gap of the terahertz comb can be interpolated while gradually shifting the high-quality comb mode little by little with high accuracy.

Moreover, in the signal waveform measuring means of the terahertz spectrometer of the present invention, it is preferable that the terahertz electromagnetic wave pulse train simultaneously acquires the signal waveform of the terahertz electromagnetic wave pulse of 5 to 10,000 cycles.
This makes it possible to observe the terahertz comb and narrow the comb mode line width. As the period increases, the narrowing can be achieved. In that case, there is a trade-off that the memory capacity as the storage means becomes enormous.

In the terahertz spectrometer of the present invention,
(E) Perform time axis scale conversion of the time waveform signal output from the signal waveform measurement means, and perform frequency analysis information of the measurement sample from the Fourier spectrum (amplitude and phase frequency spectrum) obtained by performing Fourier transform on the time axis signal. It is preferable to further include a signal analysis means to obtain.
Thereby, substance identification based on the absorption spectrum (fingerprint spectrum) peculiar to a substance can be performed.

In the terahertz spectrometer of the present invention, the mode-locked frequency tuning control means for controlling the two femtosecond laser means preferably controls the mode-locked frequency of each laser independently. Alternatively, one laser may control the mode-locking frequency, and the other laser may control the difference frequency between the two lasers to be constant.
Further, while the mode-locked frequency of one laser is continuously scanned at a high speed, the other laser may control the difference frequency between the two lasers to be constant. In this case, since the laser that continuously scans the mode locking frequency at a high speed is not stabilized, the mode locking frequency is preferably monitored by a frequency counter.

  According to the present invention, it is possible to further improve the spectral resolution. Further, according to the present invention, it is possible to improve the substance identification sensitivity based on the absorption spectrum (fingerprint spectrum) unique to the substance. Furthermore, according to the present invention, there is an effect that can be used as a frequency standard metric in the terahertz band.

Configuration diagram of measuring apparatus using THz-TDS method based on conventional mechanical time delay scanning Schematic diagram showing how a terahertz time waveform is reproduced by performing time-delayed scanning with a mechanical stage in a conventional measuring apparatus using the THz-TDS method based on mechanical time-delayed scanning Frequency spectrum of the amplitude of the terahertz electric field obtained by performing a Fourier transform with a computer from the measured time waveform of the terahertz electric field Configuration diagram of the previously proposed asynchronous optical sampling terahertz spectrometer Schematic diagram showing how the terahertz time waveform is reproduced in the previously proposed asynchronous optical sampling terahertz spectrometer Terahertz electric field time measurement waveform obtained by the proposed asynchronous optical sampling terahertz spectrometer Spectrum of terahertz comb obtained by the proposed asynchronous optical sampling terahertz spectrometer Electric time waveform of single terahertz pulse and terahertz continuous spectrum waveform obtained by Fourier transform Electric field time waveform of terahertz pulse train and terahertz discrete multispectral (terahertz comb) waveform obtained by Fourier transform Principle diagram of interpolation of terahertz comb gap by comb mode scanning Configuration of Terahertz Spectrometer of the Present Invention (Part 1) Configuration of Terahertz Spectrometer of the Present Invention (Part 2) Configuration diagram of the terahertz spectrometer of the present invention (part 3) 1 is a block diagram of the overall configuration of the terahertz spectrometer according to the first embodiment. Waveform of Terahertz Pulse Train Obtained by Terahertz Spectrometer of Example 1 Spectrum of terahertz comb obtained by the terahertz spectrometer of Example 1 Spectrum of the terahertz comb obtained by the terahertz spectrometer of Example 1 (enlarged in the vicinity of 0.5 THz in frequency) Spectral waveform synthesized by shifting the comb mode by one step by laser control

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The scope of the present invention is not limited to the following examples and illustrated examples, and many changes and modifications can be made.

FIG. 11 shows a configuration diagram of the terahertz spectrometer of the present invention. The mode synchronization frequencies of the femtosecond laser 1 (mode synchronization frequency = f ₁ ) and the femtosecond laser 2 (mode synchronization frequency = f ₂ ) are constant or tuned, and the difference between the mode synchronization frequencies (= f ₂ −f ₁ ) Stabilization control of the mode-locked frequency of both lasers is performed independently so that ₁ ) becomes a constant value. Alternatively, as shown in FIG. 12, the mode locking frequency (f ₁ ) of the femtosecond laser 1 is constant or tuned while the mode locking frequency difference between the two lasers is constant. The synchronization frequency (f ₂ ) may be controlled to be stabilized. Alternatively, as shown in FIG. 13, the mode locking frequency (f ₁ ) of the femtosecond laser 1 is tuned at high speed, while the mode locking frequency of the femtosecond laser 2 is constant so that the difference between the mode locking frequencies of both lasers is constant. (F ₂ ) may be subjected to stabilization control. In this case, since f ₁ is not stabilized, it is necessary to monitor the mode locking frequency in a frequency counter.

  The femtosecond laser 1 is used as pump light, and the femtosecond laser 2 is used as probe pulse light. The pump light emitted from the femtosecond laser 1 passes through the terahertz generating element and becomes a terahertz pulse. Further, the probe pulse light and the terahertz pulse emitted from the femtosecond laser 2 pass through the terahertz detection element, whereby the signal waveform of the terahertz pulse is measured.

In the terahertz spectrometer of the present invention, the measurement time window is widened, and the electric field time waveform of the terahertz pulse train composed of a plurality of continuous terahertz pulses is acquired by a high-speed digitizer or the like. The acquired electric field time waveform is Fourier transformed to obtain a terahertz comb spectrum waveform. The larger the measurement time window of the measured electric field time waveform, the narrower and sharper the mode line width of the terahertz comb. The acquired terahertz comb spectrum is stored in the memory.
Further, in the terahertz spectrometer of the present invention, the difference between the mode synchronization frequencies of the femtosecond laser 1 and the femtosecond laser 2 is kept constant by using the laser control method shown in FIGS. 11, 12, and 13. The mode synchronization frequency tuning control that can shift each mode synchronization frequency is performed.
With the mode-synchronized frequency tuning control, a spectrum obtained by laterally shifting each comb mode of the terahertz comb can be acquired and similarly stored in the memory. Then, a plurality of acquired terahertz combs are combined to generate an ultrafine terahertz spectrum in which the gaps of the terahertz combs are interpolated at the comb mode line width interval.

FIG. 14 is an overall block diagram of the terahertz spectrometer according to the first embodiment.
In the terahertz spectrometer of Example 1, the femtosecond laser 1 (mode-locked erbium fiber laser, center wavelength 1550 nm, mode-locked frequency (f ₁ ) 56.124 MHz, pulse width 50 fs) and femtosecond laser 2 (mode-locked erbium) A fiber laser, a center wavelength of 1550 nm, a mode synchronization frequency (f ₂ ) of 56.124 MHz + 2 Hz, and a pulse width of 50 fs) are used.
The mode locking frequencies of the femtosecond laser 1 and the femtosecond laser 2 are almost the same. A femtosecond laser 1 was used to generate terahertz pulses (pump light), and a femtosecond laser 2 was used as probe pulse light for detecting terahertz pulses. Then, stabilization control was performed such that the difference between the mode synchronization frequency and the mode synchronization frequency of both lasers became a constant value. Furthermore, the mode locking frequency (f ₁ ) of the femtosecond laser 1 and the mode locking frequency (f of the femtosecond laser 2 are controlled while performing stabilization control so that the difference between the mode locking frequencies of the two lasers (= 2 Hz) becomes a constant value. Tuning control was performed while synchronizing ₂ ).

Specifically, the stabilization control of the mode-locked frequency difference between the two lasers uses two oscillators detected by a photodetector, such as an oscillator (voltage controlled oscillator, frequency synthesizer, etc.), as shown in FIG. By performing heterodyne detection, the higher-order harmonic component signal of the mode synchronization frequency is beat-down to the beat signal in the RF frequency band. Using the beat signal of the femtosecond laser 1, stabilization / tuning control of the mode synchronization frequency (f ₁ ) is performed. On the other hand, using the beat signal of the femtosecond laser 2, stabilization / tuning control of the mode synchronization frequency (f ₂ ) is performed. That is, the fluctuation of the mode synchronization frequency and the mode synchronization frequency difference is expanded by the order of the higher order harmonics, and this is used as a control signal, thereby realizing high accuracy of the stabilization control.

In the mode-synchronized frequency scanning type asynchronous optical sampling light source control unit of FIG. 14, a value (for example, 10 MHz) having a control signal (difference frequency signal of higher harmonics) with a signal supplied from the reference signal generator as a reference signal. in to be constant, performs laser resonator length controlled by the resonator length variable element of femtosecond laser 1 (such as a piezoelectric element or stage), mode locking frequency (f ₁₎ stable to be constant at a certain value It has become. Further, the femtosecond laser 2 is also controlled so that the resonator length is controlled using the same method, and is stabilized so that the difference between the mode synchronization frequency (f ₂ ) and the mode synchronization frequency is constant at a certain value.

  A part of both of the laser beams subjected to stabilization control is a SFG (sum frequency generation light) cross-correlation measuring unit, which generates a trigger signal serving as a time origin signal. The SFG cross-correlation measurement unit condenses the two laser beams onto a nonlinear optical crystal using a lens, photoelectrically detects the SFG cross-correlation signal beams of both laser beams with a photodetector, and converts the weak current signal into current-voltage conversion. After being amplified by the amplifier, it is used as the time origin signal of the high-speed digitizer.

As shown in FIG. 14, the remainder of both laser beams is guided to a terahertz spectrometer, and the femtosecond laser 1 is used as terahertz generating pump light and the femtosecond laser 2 is used as terahertz detection probe light. A dipole photoconductive antenna was used to generate and detect terahertz.
Here, because the mode-locked frequency of the terahertz pulse generated by the femtosecond optical sampling light source and the probe pulse light is slightly different, the electric field time waveform of the terahertz pulse train can be measured at high speed without mechanical time delay scanning. It becomes.

  The current signal sampled at high speed by the photoconductive antenna for terahertz detection is amplified by a current-voltage conversion amplifier, and then measured by a high-speed digitizer using the time delay signal generated by the SFG cross-correlation measuring unit as a synchronization signal. The electric field time waveform of the terahertz pulse train is Fourier transformed to obtain a terahertz comb spectrum waveform. The comb mode is scanned while changing the mode synchronization frequency, and the terahertz comb at each comb mode position is displayed in an overlapping manner to interpolate the gap of the terahertz comb.

  FIG. 15 shows an electric field time waveform of ten series of terahertz pulse trains actually acquired. If an electric field time waveform of such a measurement time window is to be obtained by a conventional mechanical stage type THz-TDS, a mechanical stage having a length of 27 meters is required, which is practically impossible. FIG. 16 shows an amplitude spectrum obtained by Fourier transforming the electric field time waveform of FIG. The reason why the inside of the amplitude spectrum of FIG. 16 is filled is that 10,000 or more terahertz comb modes are densely packed therein.

  FIG. 17 is an enlarged view of the vicinity of 0.5 THz. From FIG. 17, it is possible to confirm the comb mode (line width 5.6 MHz) arranged at equal intervals at the mode synchronization frequency (56.1 MHz). In FIG. 17, since each mode of the terahertz comb is distributed discretely, the spectrum resolution when the terahertz comb is used for the terahertz spectroscopic measurement is a mode synchronization frequency that is a comb interval. The line width is even narrower than the comb spacing. If the terahertz comb gap can be interpolated while slightly shifting the comb mode little by little with high accuracy, an ultrafine terahertz spectrum equal to the line width of the comb mode can be obtained. The result of actually shifting the comb mode by laser control is shown in FIG. FIG. 18 shows the result when the mode synchronization frequency is shifted by 0.002% (= 1122.5 Hz). However, since the frequency shift of the mode synchronization frequency is possible up to about 1%, It is easy to fully interpolate the comb gap. In this way, an ultra-high resolution terahertz spectroscopy capable of fully covering a broadband terahertz region while having a spectral resolution equal to the line width of the comb mode is realized.

The terahertz spectrometer of the present invention can be used for qualitative and quantitative analysis of various substances exhibiting a terahertz fingerprint spectrum. It can also be used as an atmospheric environment analyzer for various gas molecules and volatile organic compounds in the atmosphere. Furthermore, it may be used as a frequency standard measuring device or terahertz tomography.

Claims (9)

The two femtosecond laser means having slightly different repetition frequencies (mode synchronization frequencies) of the laser pulse and the two femtosecond laser means are stabilized in the mode synchronization frequency, and the difference between the mode synchronization frequencies is predetermined. Using mode-locked frequency control means for controlling two femtosecond laser means so as to maintain a constant value of the above, and using the output light of one femtosecond laser as excitation light, using a photoconductive antenna or a nonlinear optical crystal, Terahertz wave radiating means for emitting terahertz electromagnetic wave pulses, and a terahertz wave optical system for irradiating a terahertz electromagnetic wave pulse emitted from the terahertz wave radiating means to a measurement sample and leading the terahertz electromagnetic wave pulse affected by the measurement sample And the output light of the other femtosecond laser as the probe pulse light. Terahertz wave detecting means for detecting the electric field time waveform of the terahertz electromagnetic wave pulse by using a photoconductive antenna or an electro-optic sampling method, and an output of the two femtosecond laser means. Extracts a part of the light and amplifies the trigger signal generator that generates the time origin signal and the weak electrical signal output from the terahertz wave detector, and detects the signal waveform of the terahertz electromagnetic wave pulse in synchronization with the time origin signal A terahertz spectroscopic measurement device comprising: a signal waveform measuring means for measuring a signal waveform by
a) The signal waveform measuring means obtains an electric field time waveform of a plurality of continuous terahertz electromagnetic wave pulses instead of a single terahertz electromagnetic wave pulse, and Fourier transforms the electric field time waveform into the frequency domain, thereby obtaining a mode-locked frequency A spectrum acquisition means for acquiring a terahertz discrete multispectrum in which frequency mode sequences consisting of fundamental wave components and multiple harmonic components are arranged at equal intervals;
b) mode-synchronized frequency tuning control means for shifting the mode-synchronized frequency of each of the two femtosecond laser means while maintaining a predetermined constant value of the mode-synchronized frequency difference;
c) storage means for storing each terahertz discrete multispectrum when the terahertz discrete multispectrum is frequency-shifted in multiple stages so as to complement the gap of the frequency mode sequence by the mode-synchronized frequency tuning control means;
d) means for synthesizing each terahertz discrete multispectrum stored in the storage means to generate an ultrafine terahertz spectrum;
A terahertz spectroscopic measurement device characterized by comprising:   2. The terahertz spectrometer according to claim 1, wherein in the mode-locked frequency tuning control means, the shift amount of the mode-locked frequency is a spectrum line width of a frequency mode in the terahertz discrete multispectrum.   2. The terahertz spectrometer according to claim 1, wherein the two femtosecond laser means use a fiber laser, a titanium sapphire laser, or a neodymium glass laser.   2. The terahertz spectroscopic measurement apparatus according to claim 1, wherein, in the signal waveform measurement unit, the terahertz electromagnetic wave pulse train simultaneously acquires a signal waveform of a terahertz electromagnetic wave pulse having a period of 5 to 10,000 cycles. In the terahertz spectrometer according to any one of claims 1 to 4,
(E) Frequency analysis of the measurement sample from a Fourier spectrum (amplitude and phase frequency spectrum) obtained by performing time-axis scale conversion of the time waveform signal output from the signal waveform measurement means and Fourier-transforming it. A terahertz spectroscopic measurement apparatus, further comprising signal analysis means for obtaining information.   The terahertz spectroscopic measurement apparatus according to claim 1, wherein the mode-locked frequency tuning control means for controlling the two femtosecond laser means controls the mode-locked frequency of each laser independently.   In the mode-locked frequency tuning control means for controlling the two femtosecond laser means, one laser controls the mode-locked frequency, and the other laser controls the difference frequency between the two lasers to be constant. Item 12. A terahertz spectrometer according to Item 1.   In the mode-locked frequency tuning control means for controlling the two femtosecond laser means, while continuously scanning the mode-locked frequency of one laser at a high speed, the other laser controls the difference frequency between the two lasers to be constant. The terahertz spectroscopy measuring apparatus according to claim 1. The terahertz spectrometer according to claim 8, wherein the mode-locked frequency is monitored by a frequency counter.